Overview of Inorganic Electrolytes for All-Solid-State Sodium Batteries

All-solid-state sodium batteries (AS3B) emerged as a strong contender in the global electrochemical energy storage market as a replacement for current lithium-ion batteries (LIB) owing to their high abundance, low cost, high safety, high energy density, and long calendar life. Inorganic electrolytes (IEs) are highly preferred over the conventional liquid and solid polymer electrolytes for sodium-ion batteries (SIBs) due to their high ionic conductivity (∼10–2–10–4 S cm–1), wide potential window (∼5 V), and overall better battery performances. This review discusses the bird’s eye view of the recent progress in inorganic electrolytes such as Na-β”-alumina, NASICON, sulfides, antipervoskites, borohydride-type electrolytes, etc. for AS3Bs. Current state-of-the-art inorganic electrolytes in correlation with their ionic conduction mechanism present challenges and interfacial characteristics that have been critically reviewed in this review. The current challenges associated with the present battery configuration are overlooked, and also the chemical and electrochemical stabilities are emphasized. The substantial solution based on ongoing electrolyte development and promising modification strategies are also suggested.


■ INTRODUCTION
−16 In comparison to the counterpart lithium metal, 17 sodium metal inherits a similar electrochemical characteristic (i.e., a specific capacitance value of 1166 mA h g −1 and a standard reduction potential of −2.71 V versus a standard hydrogenbased electrode).−21 In the current context, AS 3 B employing solid-state electrolytes (SSEs) extensively addresses the safety hazards and enables the reliable utilization of a high electrochemical potential window containing cathode materials and metal-based anodes contributing to ultralong operation and excellent energy density.Trending AS 3 B demonstrates significant characteristics such as high ionic conductivity (∼10 −4 to 10 −3 S cm −1 ), negligible electronic conductivity (∼10 −12 S cm −1 ), low self-discharge, high thermal stability, variable working temperature, a wide electrochemical potential window, and inertness toward shocks and vibrations as shown in Figure 1.A typical configuration of AS 3 B consists of a Na metal anode, SSE, and an electrolyte-contained cathode. 22,23As the critical component of AS 3 B, the solid electrolyte makes an indispensable contribution by serving as both an excellent ionic conductor and a separator between the electrodes.−28 In general, inorganic electrolytes for AS 3 B are very safe and reliable as they inhibit dendrite formation due to their absence of leakage.They offer high thermal stability, a nonvolatile nature, and ease of design, whereas liquid electrolytes may rapidly catch fire in nonaqueous-based solvents.Furthermore, they also exhibit high ionic conductivity (10 −2 to 10 −3 S cm −1 ) and a wide electrochemical stability window (∼5 V) with a high energy density.Solid polymer electrolytes, on the other hand, have low ionic conductivity (10 −4 to 10 −6 S cm −1 ) and and a small electrochemical stability window (ESW, ∼3.4 V) due to their high ionic transfer resistance.IEs show highly stable structures with a wide temperature range (−70 to 500 °C), whereas high interfacial resistance was developed between electrolyte layers for the solid as well as composite polymer electrolytes (CPE).Similarly, the aggregation of inorganic particles over a polymer has a greater disadvantage for solid CPE.Overall, the inorganic electrolytes lead to higher storage life and better battery performance compared to all other solid-state electrolytes. 29,30his review mainly focuses on the progress of inorganic electrolytes for AS 3 B.There are several review papers in this domain.However, their primary focus is on the development and categorization of inorganic materials.Hence, recent advances in improving the ionic conductivity, ESW of ISE, and electrode−electrolyte interface mechanism have not yet been reviewed. 5,26Herein, we have summarized solely the development of inorganic electrolytes by emphasizing their classifications and respective mechanisms for ionic conduction.Additionally, strategies to enhance recent progress in the improvement of the interfacial characteristics, which include cathode−electrolyte and anode−electrolyte aspects, are critically assessed.Furthermore, the mechanism of dendrite formation and the possible solution are also addressed.The current challenges associated with the present battery configuration involve emphasizing the chemical and electrochemical stabilities.A substantial solution based on the ongoing electrolyte development and promising modification strategies is also suggested.
■ BOTTLENECK OF ALL-SOLID-STATE SODIUM BATTERIES Undesirable Volume Changes during Plating and Stripping Processes.It is noteworthy that a traditional intercalation-type electrode such as graphite undergoes a 10% volume change to accommodate the ions on the anodic side during the intercalation and deintercalation processes.Current AS 3 Bs employed metallic sodium as the anode that undergoes a much greater volume change compared to the graphite during the electrochemical plating and stripping process.This uncontrollable volume expansion led to the destruction and reformation of the solid electrolyte interfacial layer at the anode side upon successive insertion and disinsertion of Na + in AS 3 Bs.Thus, inevitable reversible volume exchange during the plating/ stripping processes led to the induction of mechanical stress, which in turn led to the electrochemical pulverization of the electrode material.This adversely contributes to the capacity fading in current AS 3 Bs.Comprehensive effort is directed toward the mitigation of undesirable volume expansion for the long-term cyclic operation in AS 3 Bs.
Stabilization of Interfaces.In AS 3 B, achieving a stable solid−solid interface between the Na metal and inorganic ceramic electrolyte is a major hurdle in the realization of electrochemically stable and high-performing energy storage.The Na metal, being highly reactive, effectively reduces the IEs and consequently destabilizes the Na−IEs interface, degrading the device performance.Particularly in AS 3 B, an ideal solid electrolyte interface (SEI) must inherit the following characteristics: • SEI should be highly ionically conductive and act as an electronic insulator across the electrode−electrolyte interface to ensure uniform Na + deposition and thereby reduce the preferential Na deposition.
• SEI should be electrochemically and chemically inert to prevent undesirable chemical reactions with the electrolyte deteriorating the electrochemical performance.
• SEI should be mechanically robust to ensure successive volume change and dendrite propagation during the deposition and stripping process in AS 3 B.
Inevitable subpar electrochemical cyclic performance and low Columbic efficiency due to unstable SEI formation at the electrode−electrolyte interface are effectively addressed by introducing intimate contact across the interface.Intimacy of contact at the interface is assessed by measuring the area-specific resistance (ASR) that substantially ensures the homogeneous stripping and deposition of Na + ions during the operation in AS 3 B. The ASR across the sodium metal and IEs can be determined using Equation 1 where ASR, R inter , and S refer to the area-specific resistance, internal resistance offered by the fabricated cell, and effective contact area of the metallic Na−IE interface, respectively.The effective contact area in the area-specific resistance (ASR) refers to the interface between the inorganic electrolyte and the electrode.The contact area is a crucial factor in reducing the ASR.A lower ASR leads to better battery performance.In this regard, a larger contact area allows for better electron and ion transfer between the inorganic electrolyte and the electrode, leading to lower resistance and enhanced battery efficiency.Also, reaching an ASR electrolyte of 24 Ω cm 2 is the highest ASR cell value for commercial 18650LIBs. 31In the case of the symmetric cell configuration where the same contact area is involved in ASR, the measured R inter is halved to evaluate the ASR value.A large value of ASR results in an increasingly high overpotential that ultimately reduces the energy efficiency and dendrite formation.This adversely leads to the inhomogeneous nucleation in the Na + ions in the plating process and thus increases the local current density.Therefore, developing a good interface between the electrode and the inorganic electrolyte, optimizing the electrode−electrolyte interface, and reducing the space between the electrode and the electrolyte can help to reduce the interfacial resistance and improve the battery's performance.Dendrite Formation at the Metal Anode.The dendritic growth of Na metal in SSE is due to volume change, chemical instability, and mechanical incompatibility.In general, the dendrite growth in a liquid electrolyte system occurs when Na metal undergoes directional dendrite formation.Apparently, the LUMO level of the liquid electrolyte is below the redox potential of Na, which reductively decomposes the electrolyte until the formation of a solid electrolyte interface (SEI) layer.In contrast, dendrite growth in SSE undergoes two mechanisms.The first is similar to the liquid electrolyte�directional dendrite forma-tion�and the latter is the dispersed Na dendrite plating within the SSE.The plating of Na ions occurs in high-energy areas such as grain boundaries, voids, and defects within the SSE.Also, when diffusing through the SSE, the Na + ions are reduced by the leaking electrons from these areas (Na + → Na 0 ).Thus, dendrites grow and fill up the pores in subsequent cycles.After all of the pores are filled up, the dendritic growth commences at the grain boundary, leading to a detrimental effect on the battery operation.However, compositional elements of some inorganic SSE decrease with the Na metal to form SEI layers (e.g., Na 2 O, NaI, NaCl, etc.).These layers suppress the contact between the metal anode and high energy areas of inorganic SSE, reducing the dendrite growth. 32urthermore, the sodium metal inherently shows a higher chemical reactivity and weaker mechanical structures.This is due to the larger atomic radius and weaker metallic bonding in sodium metal than those of the lithium metal.Like the conventional LIBs, AS 3 Bs are plagued by the growth of dendrite upon successive electrochemical plating and striping processes, thus giving birth to safety hazards.The nonuniform deposition at metallic sodium led to the formation of sodium dendrites and cavity formation.From the thermodynamical concept, the formation of dendrites occurs when the electrochemical potential is lower than the standard electrode potential of metals (−2.71V vs SHE for Na/Na + ).Consequently, conventional Na-ion batteries rarely encounter the issue of dendrite formation because of the higher working potential of intercalation-based anodes (e.g., 2.84 V vs SHE for graphite) compared to that of metals.The possible causes of the dendrite's formation in conventional metal-ion batteries are the concentration profile influence of standard organic liquid electrolyte with sodium metal, the instability of the plane interface, and nonuniformity in the electrode−electrolyte interface.Herein, the term "plane interface" refers to the boundary or contact surface between two different materials present within the battery system.These interfaces play a critical role in determining the fundamental aspects of electron transfer and ion transport, which are essential to the overall performance of the battery.One possible cause of dendrite formation is the instability of the plane interface, which can be influenced by the mass transport of Na ions in the liquid electrolyte and the SEI layer.If the mass transport is nonuniform, then it can lead to an uneven deposition of Na.Additionally, the SEI layer can be unstable and can undergo continuous formation and dissolution, leading to the formation of dendrites.These are the factors causing the formation of dendrites in AS 3 B.

■ IONIC CONDUCTION MECHANISM
The pursuit of high ionic conductivity renders the development of highly ionically conductive IEs.A preferred IE must be an ionic conductor inheriting negligible electronic conductivity.Extensive effort has been expended in the exploration of advanced electrolyte materials and the modification strategies (i.e., doping, substitution, composite formation, coating, crystal formation, ceramization, etc.) to achieve high ionic conductivity.In IEs, ionic conduction originates through the long-range transport of mobile sodium ions via interstitial hopping with a similar energy state.Intrinsic ionic conduction relies on the availability of mobile ions/voids, the hopping vacancy sites, and the concerned energy barrier for the hopping of mobile ions.The ionic conduction is extensively influenced by the crystal structure, lattice dynamics grain boundaries, and defect structure.The responsible mechanism in IEs is observed mainly in three ways: (1) vacancy diffusion, (2) interstitial site migration, and (3) knock-on or correlation-based movement through grain boundaries.The ionic conductivity (σ) of IEs is governed by the Arrhenius equation as shown in Equation 2: As evident from the above equation, σ is the product of charge (q), concentration of mobile ions (n), and ionic mobility (μ) of the charge carrier of an IE.Ionic conduction in a thermally activated process that obeys the typical Arrhenius equation, in which k B refers to the Boltzmann constant, T refers to temperature, and E a refers to the characteristic activation energy required for ion conduction.In the case of IEs, high concentration of mobile ions (n) and lower activation energy (E a ) are the crucial factors in achieving high ambient temperature σ. 33 Ionic Conduction Mechanism of Oxide-Based Electrolytes.The ionic transport mechanism of Na-β-Al 2 O 3 is stated on account of its crystal framework is shown in Figure 2a.It is evident from the chemical structure that Na-β′′-Al 2 O 3 is composed of basic building blocks of alternatively arranged layers with spinel blocks and a conduction plane.Specifically, the spinel block is composed of four layers of stacked oxygen ions surrounded by aluminum ions accompanied by the conduction planes comprising packed oxygen and sodium ions.In the case of its two polymorphs (i.e., Na-β′′-Al 2 O 3 and Na-β-Al 2 O 3 ), the ionic conduction occurs solely through the conduction plane and there is a negligible contribution of vertical stacks.In fact, both βand β″-alumina formed the tetrahedral and octahedral Al−O spinel blocks.In particular, the β-phase has a hexagonal lattice representing the space group P6 3 /mmc, whereas the β″phase has a rhombohedral lattice with a space group of R3̅ m.The unit cell of the β-phase contains one Na ion, whereas the β″phase has two Na ions in each of the conduction layers.Therefore, Na-β′′-Al 2 O 3 (0.35 S cm −1 ) with a higher Na + concentration and a larger unit cell with dissimilar oxygen stacking sequences serves as a better ionic conductor than Na-β-Al 2 O 3 (0.3 S cm −1 ). 34milarly, NASICON's structural framework plays a vital role in facilitating appreciable ionic transport.NASICON comprises a covalent three-dimensional framework structure with corners shared by MO 6 octahedra and PO 4 tetrahedra that constitute a skeletal structure with a high availability of interstitial sites as 3D interconnected Na-ion migration pathways, as well shown in Figure 2b.NASICONs can form three types of crystal structures (i.e., rhombohedral, monoclinic, and triclinic based on composition). 35In the rhombohedral crystal structure of Na 1+x Zr 2 Si x P 3 O 12 (0 ≤ x ≤ 3), Na + resides on two sites for x< 0. The large number of mobile Na + and the adjacent vacancies can coexist in the rhombohedral phase, which is very beneficial for Na + diffusion.At the same time, a deformed monoclinic phase would form a less symmetrical structure that may be conducive to Na + migration.The migration of Na + occurs from Na(1)−Na(2) channels via successive ion migration, as shown in Figure 2b.On the other hand, the splitting of Na(2) sites into another Na(3) site takes place in the monoclinic phase.Consequently, four bottleneck channels are created (i.e., two from Na(1)−Na(2) and the other two from Na(1)−Na(3)). 35n case of P-2-type layered materials (i.e., Na 2 Cu 2 TeO 6 , Na 2 Ni 2 TeO, Na 2 Zn 2 TeO, and Na 2 Mg 2 TeO 6 etc.), there exists a P-2 type Na x CoO 2 structure with a complete M 2+ /Te 6+ ordering in each (MO 2 ) n layer. 36,37These P-2-type layered structures exists in different polymorph hexagonal space groups (i.e., P6 3 / mcm (Na 2 Ni 2 TeO 6 ) and P6 3 22 (Na 2 Zn 2 TeO 6 / Na 2 Co 2 TeO 6 )) based on the in-plane shift of the (M 2 Te) n layers. 38Basically, there are three crystallographic sites evident from the quantitative analysis of X-ray diffraction, where Na + resides in Na 2 M 2 TeO 6 (i.e., Na1(6g), Na2(2a), and Na3(4f)).Theoretical studies such as molecular dynamics (MD) simulation on the Na + ion transport in Na 2 Ni 2 TeO 6 revealed that the hopping of Na + ions occurs dominantly through Na1 to Na2, as compared to the Na3 site, as shown in Figure 2c.This is due to the fact that Na3 crystallographic sites (−2.32 eV) have a higher potential energy than Na1 and Na2 sites (−2.45 and −2.65 eV, respectively). 39Additionally, the bond valence sum (BVS) studies stated that Na3 inherits the same potential energy values as Na1 and Na2 in Na 2 Zn 2 TeO 6 due to the lesser repulsion of Na + with Zn 2+ in the NZTO framework than that of Na + with Te 6+ in Na 2 Ni 2 TeO 6 .
In the case of the layered oxides, the larger spacing of Na 2 Zn 2 TeO 6 is due to the closest oxygen atoms in adjacent layers, which share the same (x, y) coordinates.The important factor is mainly the O−O distance rather than the layer separation, and they are found to be 3.42 and 3.58 Å for P2 and O′3, respectively.The shorter O−O distance in the P2-type Na 2 Zn 2 TeO 6 causes additional repulsion between the layers and increases the ionic conductivity.NZTO has a capability to enlarge the space between the strongly bonded 2D layers as shown in Figure 2c.In this way, a greater number of Na + ions gain a spacious interstitial migration path comfortably than the equivalent close-packed inorganic oxide-based electrolytes.Considerable attention has been paid to the insight into the intercalation and diffusion of Na ions existing in the lamellar oxide system.Among all of the investigated Na 2 M 2 TeO 6 , where M = Cu, Ni, Zn, or Mg, Na 2 Zn 2 TeO 6 demonstrated excellent ionic conductivity owing to the largest interlayer space (∼5 Å), which provides a large Na + ion migration path as compared to the other analogues. 36−42 This type of substitution triggers the ion transport in two ways: (1) increasing the charge carrier density and (2) enlarging the ionic migration pathway in Na 2 Zn 2 TeO 6 .
Ionic Conduction Mechanism of Sulfide-Based Electrolytes.Sodium superionic conductors mainly refer to the sodium thiophosphates, and considerable attention has been paid to AS 3 B applications owing to their excellent ionic conductivity and high ductility.A class of lithium analogue sulfides such as Na 3 PS 4 , Na 11 Sn 2 P 2 S 12 , Na 3 SbS 4 , etc. have emerged as excellent ionic conductors. 43The high σ is devoted to the high availability of vacant sites in the crystal framework contributing to the hopping of ions.This prerequisite for high ionic conduction varies from structure to structure.Generally, Na 3 PnS 4 (where P = Sb, Sn) crystallizes in two forms (i.e., cubic (space group I43m) or tetragonal (space group P421c)) as shown in Figure 2d. 44Primarily, cubic Na 3 PnS 4 contains a bodycentered cubic (BCC) arrangement of PnS 4 3− units, whereas the tetragonal Na 3 PnS 4 shows a rotation of PnS 4 3− around the [111]  axis that goes along with a minor tetragonal distortion along the c axis.This results in the availability of one and two Na + ions in the cubic and tetragonal polymorphs of Na 3 PnS 4 , respectively.The ion migration pathway of fully occupied Na + is shown in Figure 2d.In the case of full occupation, the Na + transport properties completely depend on the concentration defect, synthesis parameters, and crystal structure.
Similarly, as derived from Li 10 GeP 2 S 12 , Na 10 Sn 2 P 2 S 12 is reported to be an excellent sodium-ion conductor for AS 3 B. It crystallizes into the tetragonal phase (space group, I4 1 /acd) that features a peculiar three-dimensional chessboard-like framework of SnS 4 and PS 4 tetrahedra that expands the Na + ion channels along the c axis and within the ab planes, as well depicted in Figure 2e.Ab initio molecular dynamics (AIMD) simulation for Na 11 Sn 2 P 2 S 12 showed that the Na + ions occupy only octahedral sites, and ionic conduction is solely three-dimensional in nature.

■ INORGANIC ELECTROLYTES
Inorganic electrolytes (IEs) are the crucial component of AS 3 B like the electrode materials.The selection of suitable IE solely determines the practical characteristics of batteries such as power and energy density, ultralong stability, and safety standards.For practical AS 3 B, an ideal electrolyte must inherit the following traits: • IEs should have excellent ionic conductivity of ∼10 −2 S cm −1 at different operating temperatures (ambient to moderate).
• IEs should have negligible electronic conductivity.
• IEs should be thermally stable.
• IEs should have a wide electrochemical potential stability window of ≥5 V vs Na + /Na (to increase the cell voltage and to eliminate the unwanted interfacial reactions with high-voltage cathodes and Na metal anodes).• IEs should be chemically inert toward the Na metal anode and Na-containing cathode.• IEs should be dense and mechanically strong, capable of suppressing dendrite formation.• IEs should be easily processable into thin film.
NASICON-Type Electrolytes.In 1976, Hong and Goodenough introduced NASICON (a Na super ionic conductor) with the chemical formula Na 1+x Zr 2 Si x P 3 O 12 (0 ≤ x ≤ 3) derived from parent compound NaZr 2 P 3 O 12 with the partial replacement of P 5+ sites by Si 4+ .The Na 1+x Zr 2 Si x P 3 O 12 electrolyte with x = 2, Na 3 Zr 2 Si 2 PO 12 exhibited the highest σ (6.7 × 10 −4 S cm −1 @ 25 °C and 2 × 10 −1 S cm −1 @ 300 °C).Na 1+x Zr 2 Si x P 3 O 12 (1.8 ≤ x ≤ 2.2) generally crystallizes into polymorphs (i.e., monoclinic (space group C2/c) and rhombohedral (space group, R3c) crystal structures), as shown in Figure 4(a and b).The two phases comprise SiO 4 tetrahedra, PO 4 tetrahedra, and ZrO 6 octahedra positioned at the corners, creating a "hexagonal bottleneck" for Na + migration and having the shortest diameter of 4.6 Å with the anion framework consisting of four different sites for cations. 51The large space in the crystal framework facilitates abrupt Na + migration through the bottleneck, thus increasing the σ.In the rhombohedral crystal structure, sodium resides in two different sites (i.e., Na1 and Na2), forming a threedimensional diffusion network, whereas in the distorted monoclinic phase, original Na2 splits into Na2 and Na3 sites to form two distinct migration pathways (i.e., Na1−Na2 and Na1−Na3, respectively).The Na2 site is capable of accommodating 3 mol of Na + in the rhombohedral phase space. 52he substitution of a low-valence ion in the NASICON crystal framework modifies the composition by introducing more Na + into the lattices, causing an increased density of mobile sodium ions in accordance with the charge balance principle.Thus, a significant improvement in σ is observed for substituting the Na site (Li + , K + ), Zr/Si site (Cu 2+ , Mn 2+ , Ca 2+ , Zn 2+ , Al 3+ , Mg 2+ , Gd 3+ , La 3+ , Nd 3+ , Sc 3+ , Er 3+ , Fe 3+ , Tb 3+ , Eu 3+ , Y 3+ , Yb 3+ , Ce 4+ , Ti 4+ , Si 4+ , etc.), and P site (Si 4+ , As 5+ , and S 6+ ).Moreover, the substitution of similar radii atoms with the parent atoms effectively widens the size of bottlenecks and lowers the activation energy for ion transport, which in turn increases the σ.
Further enhancement in σ can be achieved by improving the packing density, optimizing the secondary phase, and modifying the grain boundary. 53Recently, Ceder et al. theoretically predicted the stability rules of NASICON based on the materials discovery, physical interpretation, and machine-learning tools. 54aking into account the crystal structure of NASICON and respective Wyckoff positions for cations as shown in Figure 4(a and b), they performed phase diagram calculation analysis to determine the energy of NASICONs in the chemical space of Na-M 1 -M 2 -A-B-O.They screened over 21 metals and determined 3881 distinct compositions.It is anticipated from previous reports that the Na content (x) in the NASICON framework varies from 0 to 4, and there is a wide range for choosing the cations to coexist at the M and A sites in the NASICON framework, as depicted in Figure 4c.Indeed, multiple parameters play a role in limiting the phase stability, such as bond compatibility across different site conditions and the propensity for an element to use a specific interaction condition.Furthermore, the Na + migration barriers for NZSP were estimated to determine its coordinated structure in the dilute limit of single Na + without any other cations.The Na + migration barrier is found to be 0.44 eV, which is lower than the Li + migration barrier (0.29 eV) as shown in the relative energy graph in Figure 4(d and e).They experimentally validate the reported σ versus the Na content (x) for nearly 200 NASICON compositions, as shown in Figure 4f.It is clear that σ increases with the increase in Na content in the Na X M 2 (XO 4 ) 3 for up to X = 3, which tends to be the optimum Na-rich stochiometric composition.
Thermodynamic stability studies reveals that NASICONs containg metals such as Hf 4+ , Zr 4+ , Ta 5+ , and Sc 3+ exhibit higher stability and compatibility than other metals.However, a stabilized NASICON crystal structure is difficult to achieve in The introduction of the Mg 2+ site at the Zr 4+ site effectively resulted in a higher stripping/plating behavior over 2000 h at 0.1 mA cm −2 without any shortcircuiting. 55ulfide-Based Electrolytes.Sulfide electrolytes are the prevailing electrolytes for IEs in the AS 3 B and are highly preferred over their oxide's analogue.Owing to the advantage of excellent ionic conductivity, low synthesis temperature, superior mechanical properties, isotropic ionic conductions with negligible grain boundary resistances, reduced production cost, and an intimate contact electrode−electrolyte interface, sulfide-based oxides served as reliable electrolyte materials. 57arious types of the reported chalcogenide-based SSEs with corresponding σ are shown in Figure 5(a and b).Hyashi et al. first reported the Na 3 PS 4 -based glass−ceramic electrolyte in two crystal forms: cubic (space group, P421c; a 0 = b 0 = 0.69520 nm, c 0 = 0.70757 nm) and tetragonal phases of 75Na 2 S-25P 2 S 5 glass at 270 and 400 °C, corresponding to the cubic (P421c) and tetragonal phases (space group I43m; a 0 = b 0 = c 0 = 0.70699 nm), respectively.As an optimized superionic cubic Na 3 PS 4 crystal with the composition of 75Na 2 S-25P 2 S 5 , it exhibited a σ of 2.0 × 10 −4 S cm −1 with a potential window of about ∼5 V and an appreciable electrochemical stability against Na.Generally, sulfide-based electrolytes are categorized into glass and glassceramic electrolytes.
Both the cubic and tetragonal phases of NaPS 4 have minute differences with less than a 0.2% volume change in both phases.In the cubic polymorph, the Na resides at the Na1(6b) sites, whereas in the tetragonal phase, the Na1 sites split into Na1(2a) and Na2 (4d) sites are shown in Figure 5(c and d).The cubic  phase shows a higher σ value when compared to the tetragonal phase.
Sulfur is highly efficient in providing a wide ion-transport pathway for rapid sodium ion migration due to its negligible electronegativity with weakly bonded ions and a larger radius.The aliovalent doping with Si, Se, Sn, Ge, and S 2− in P sites generates vacancies in the Na 3 PS 4 crystal framework, significantly providing fast sodium ion migration and thus resulting in enhanced ionic conductivity.Despite its superior σ, sulfidebased electrolytes are prone to instability toward air (or moisture sensitivity).These sulfide electrolytes undergo hydrolysis when encountering moisture and thus releases noxious H 2 S gas.It is theoretically proven that Na vacancies are induced upon the doping of cations in Na 3 SbS 4 .The cation doping plays a vital role in improving σ, while the diffusion is further characterized in terms of the concerted motion of sodium ions.
Recently, Jiao et al. synthesized cubic-phase Mn-substituted Na 3 SbS 4 attributed to the low energy barrier compared to that of Na 3 SbS 4 , resulting in the high σ of 2.05 mS cm −1 with a wide electrochemical window of 5 V. Na 3.24 Mn 0.08 -Sb 0.92 S 4 with an increased crystallinity effectively inhibits the sodium dendrite growth and excellent air stability. 62 Comparatively, WS 4 /MoS 4 displays a minimal volume with an increased bond angle deviation (5.56 Å, ±0.22°) compared to that of SbS 4 (6.82Å, 0°).Similarly, the lattice parameter calculated from the DFT tends to be 7.244 Å for MoS 4 and 7.248 Å for WS 4 .Hyashi et al. also reported a highly ion-conductive Na 2.88 Sb 0.88 W 0.12 S 4 , which is the same as Li 10 GeP 2 S 12 .An unprecedented σ of 32 mS cm −1 via partially doping Sb in Na 3 SbS 4 with tungsten is anticipated for the induced Na vacancies and the transition from the tetragonal phase to the cubic phase as shown in Figure 5(g and h).This sulfide-based inorganic solid electrolyte delivers an extensive tolerance of H 2 S gas under ambient conditions, which is very helpful in improving the safety and reducing manufacturing costs.
Borohydride-Based Electrolytes.Recently, complexbased electrolytes have emerged as a promising electrolyte for AS 3   characteristic crystal structure was explored using a twodimensional magnetic resonance techniques.The XRD pattern and crystal structure of pristine NaB 11 H 14 and combined Na x+2y (B 11 H 14 ) x (B 12 H 12 ) y (x:y ratios of 2:1, 1:1, and 1:2) are shown in Figure 6(a−d).The space group obtained for 2:1 and 1:1 is Im3m, whereas for 1:2 it is Pm3n.Furthermore, the Na + per unit formula is 1.67 for the 1:2 ratio, which is greater than the other two ratios (2:1−1.33 and 1:1−1.5).Overall, the 1:2 ratio of Na 5 (  .Furthermore, layered sodium-ion conductor Na 2 NiFeTeO 6 reported the highest σ of 4 × 10 −3 S cm −1 at 300 °C.The Na 2 Zn 2 TeO 6 framework possesses a large two-dimensional reticulate Na + migration interstitial pathway between two adjacent honeycomb structure layers, thus contributing to remarkable σ. 39 Successive enhancement in σ was practiced via element doping (Mg, Ni, Ga, and Ca), 38,41,42 aiming to achieve a higher Na + concentration and enlarge the ionic transport pathway as shown in Figure 7a.Han et al. reported a galliumdoped P2-type layered material Na 1.9 Zn 1.9 Ga 0.1 TeO 6 with an unprecedented σ of 1.1 × 10 −3 S cm −1 at ambient temperature, surpassing the values achieved for NASICON and Na-β′′-Al 2 O 3based solid state electrolytes.
A S 3 B w i t h a c o n fi g u r a t i o n o f N a 3 V 2 ( P O 4 ) 3 / Na 1.9 Zn 1.9 Ga 0.1 TeO 6 /Na demonstrated excellent chemical and electrochemical performances by delivering a capacity of ∼70 mA h g −1 over 10 cycles with a current rate of 0.2 C at 80 °C as shown in Figure 7b. 68The exceptional σ was accounted for by the two reasons.The insertion of Ga 3+ into the NZTO framework led to the enlarged Na + ion migration pathway interlayers (∼5.58 Å) being more compared to NASICON and β/β″-alumina-based inorganic electrolytes.Increasing the Nasite vacancies upon the inclusion of Ga 3+ in NZTO increased the carrier's concentration and lowered the migration energy for Na + ions.The interfacial resistance of Na 3 V 2 (PO 4 ) 3 /  Na 1.9 Zn 1.9 Ga 0.1 TeO 6 /Na was found to be lower than that of the Na 3 V 2 (PO 4 ) 3 /Na 2 Zn 2 TeO 6 /Na (i.e., reduced from ∼360 to ∼225 Ω), revealing the interfacial improvement in the NZTO upon Ga 3+ substitution.
The same strategy for broadening the interlayer distance to achieve high σ was reported by Han et al.The group investigated the role of Ca 2+ substitution in different concentrations in the P2-type layered NZTO (Na 2 Zn 2−x Ca x TeO 6 (x = 0−0.05)). 42mong all investigated samples, Na 2 Zn 2−x Ca x TeO 6 with x = 0.02 exhibited a maximum Na + conductivity of 7.54 × 10 −4 S cm −1 .This is anticipated to the grain-boundary modification and interlayer-interface elimination upon insertion of Ca 2+ .Additionally, Ca 2+ substitution increases the interlayer spacing in NZTO as shown in Figure 7c.The peak shift over NZTO is due to the larger radius of Ca 2+ (0.099 nm) compared to that of Zn 2+ (0.074 nm) as depicted in Figure 7d.Furthermore, the unknown phase observed in the XRD pattern of Na 2 Zn 2−x Ca x TeO 6 with x = 0.02 is anticipated to be the modification in the element concentration of the main phase but also densifies the Na 2 Zn 2−x Ca x TeO 6 with the x = 0.02  electrolyte, leading to a reduction in the interfacial resistance.Interestingly, Fjellvag et al. investigated the theoretical and experimental insights into the crystal structure and diffusion in the biphasic NZTO crystal framework. 39They reported first the existence of O′3-type phase for NZTO.Their investigation revealed that a pure P2-type phase can be attained for synthesis temperatures below 900 °C.For temperatures above 900 °C, mixed P2 and O′3 phases were formed.The pure O′3 phase is achieved only via the insertion of Li at the Zn site in the NZTO framework (Na 2+x Zn 2−x Li x TeO 6 , x = 0.2−0.5)as shown in Figure 7e.The synchrotron combined with the computational modeling indicates the balanced Na charge content after the incorporation of Li into the Zn.Based on computational modeling-enabled impedance spectroscopic studies, they stated that a reduction in the O′3-type phase solely contributed to the enhanced σ.Comparison analysis of inorganic electrolytes in terms of ionic conductivity (Table 1) and along with electrochemical stability windows of AS 3 B are shown in Figure 8.

■ ELECTRODE−ELECTROLYTE INTERFACE
The AS 3 B fabrication comprises a solid electrolyte along with a separator layer sandwiched between compressed electrode materials (cathodes/anodes).Electrode materials are commonly composed of active materials with some conducting agents.During operation, the Na ion diffused from active electrode material to the solid state electrolyte through the interface, resulting in electron migration from active material to current collector.Thus, the charge transfer through the interface in AS 3 B involves the cathode/electrolyte and anode/electrolyte interfaces.An ideal interface should possess compatibility, good mechanical strength, high ionic conductivity, and appreciable wetting characteristics.The current AS 3 B is prone to high interfacial resistance and poor interfacial contact due to an unintended volume change of electrodes during the Na + intercalation/deintercalation, an inadequate contact area inheriting restricted ion transport pathways, and low active material loading. 100Owing to the high reactivity and large Na + radius, the interfacial issues are more challenging in AS 3 B. The currently reported electrode materials (cathode/anode) are shown in Figure 9.

■ CATHODE−ELECTROLYTE INTERFACE
The solid−solid contact in the cathode−electrolyte interface is the most vital prerequisite for achieving high-performance AS 3 B. The hard texture of active materials and balanced ionic/ electronic conductivity are responsible for the poor interfacial contact across the cathode−electrolyte interface. 101The strategies to improve the cathode−electrolyte interface are as follows: Composite Cathode Formation.The most common method for fabricating the interfacial contact is to develop a composite cathode by mixing active material and electrolyte, which has been widely used in both inorganic and organic solid electrolyte batteries.For example, Goodenough et al. proposed a NaTi 2 (PO 4 ) 3 composite cathode membrane of cross-linked poly(ethylene glycol) methyl ether acrylate (CPMEA), which served as a Na + conducting binder with carbon black as the electron conductor. 102AS 3 B with the configuration of NaTi 2 (PO 4 ) 3 , Na 3 Zr 2 Si 2 PO 12 /Na exhibited a discharge capacity of 110 mA h g −1 at a 0.2 C rate up to an initial 25 cycles and then 75 mA h g −1 at 1 C during the following 35 cycles.When the battery was tested at a C rate of 0.5 C, it delivered a discharge capacity of 94 mA h g −1 .In all three tests, the Coulombic efficiency of 99.8 ± 0.2% was retained, indicating a high sodium deposition/stripping efficiency and appreciable electrochemical stability of AS 3 B. The same group has introduced AS 3 B with a plastic−crystal electrolyte interphase of succinonitrile and NaClO 4 to improve the interfacial contact between the Na 3 Zr 2 (Si 2 PO 12 ) pellet and the Na 3 V 2 (PO 4 ) 3 cathode. 103The presence of the plastic−crystal electrolytes in the cathode forms an intimate contact with the Na 3 V 2 (PO 4 ) 3 cathode to improve the efficiency of sodium-ion transfer.Similarly, a sodiated naflon-modified composite cathode was reported to improve the cathode (Na 3 V 2 (PO 4 ) 3 /Na 3 Zr 2 Si 2 PO 12 /C, mass ratio 60:38:2)-Na 3 Zr 2 Si 2 PO 12 electrolyte interface. 104The as-fabricated AS 3 B with a sodiated naflon-modified composite cathode exhibited a discharge capacity of 81.6 mA h g −1 at 20 mA g −1 and retained a high capacity of 62.23 mA h g −1 after 50 cycles.Unlike the liquid electrolyte, the introduced sodiated nafion in the swelling state facilitated a stable and rapid migration path for ionic conduction and served as a buffer layer for undesirable volume change in the cathode as a consequence of repeatedly charging and discharging.
In another work, Yao et al. reported a composite cathode with a composition of Na 4 C 6 O 6 :Na 3 PS 4 :carbon 4:5:1 AS 3 B using Na 3 PS 4 as an electrolyte as depicted in Figure 10a. 105The cold pressing technique promotes the direct contact between the cathode and electrolyte (Figure 10b), resulting in good battery performance.The assemble battery delivered a high specific capacity and high energy density of 184 mA h kg −1 and 395 W h kg −1 , respectively.The cell shows an appreciable capacity retention of 76% for 100 cycles at 0.1 C and 70% for 400 cycles at 0.2 C, as shown in Figure 10(c−e).Furthermore, the postcycling studies also confirmed no new species formed during the battery cycling, resulting in excellent compatibility between Na 4 C 6 O 6 and Na 3 PS 4 .The effort to address the cathode electrolyte interfacial concern was made by Yao et al., in which they proposed a composite cathode material with PTO-Na 3 PS 4 -C at a weight ratio of 20:(80−x):x, where x = 0.5, 10, 20, 28, and 33.Immediate contact of PTO with Na 3 PS 4 helps the nanoparticles to bear the mechanical stress generated at the interface during the charge−discharge cycles.The as-fabricated AS 3 B delivered a specific capacity of 304 mA h g −1 at 0.1 C with an excellent capacity retention of 97% after 100 cycles.
Similarly, the composite cathodes are also reported for AS 3 B batteries with β-alumina electrolytes.For example, Honma et al. reported a Na 2 FeP 2 O 7 glass ceramic cathode in which Na 2 FeP 2 O 7 glass was successfully joined with a Na 2 OFe 2 O 3 P 2 O 5 glass substrate by pressure less heat treatment cofiring at 550 °C. 106The composition of the cathode (i.e., Na 2 OFe 2 O 3 P 2 O 5 glass, β″ alumina solid electrolyte, and acetylene black (AB)) was kept in the weight ratio of 72:25:3.The immediate contact at the cathode−electrolyte interface does not get peeled off during charging and discharging over a long period of 623 cycles.
Surface Coating.Surface coating of active material particles with a thin layer of solid electrolytes is another effective technique to form intimate interfaces.For example, solid−liquid interfaces can be achieved by heating the glass electrolytes to around the glass-transition temperature, followed by favorable contacts after cooling to ambient temperature.Wang et al. proposed a substantial solution by introducing a thin layer of Na 3 PS 4 coated on Mo 6 S 8 using a solution method to achieve intimate contact at the Na 3 PS 4 −Mo 6 S 8 interface as shown in Figure 10(f and g). 107The Na 3 PS 4 -coated Mo 6 S 8 delivers a higher first cycle revisible capacity and capacity retention than does the bare Mo 6 S 8 cathode.The as-fabricated cell containing an Na 3 PS 4 -coated Mo 6 S 8 electrode exhibited high capacities of 90, 86, 82, 67, and 61 mA h g −1 at current densities of 5, 10, 15, 30, and 60 mA g −1 , which were much higher than those of the bare Mo 6 S 8 electrode that delivered only 75, 62, 60, 48, and 33 mA h g −1 at the same currents of 5, 10, 15, 30, and 60 mA g −1 , respectively.This is due to the Na 3 PS 4 coating that improved the reaction kinetics and also minimized the interfacial resistance from 1125 to 654 Ω.Furthermore, Na 3 PS 4 plays an intermediate role in the chemical compatibility with the sulfide-based cathode, and no further chemical reactions occurred in the Na 3 PS 4 −Mo 6 S 8 interface.
In another work, Wen et al. developed a coating layer on the surface of the β′′-Al 2 O 3 solid electrolyte comprising microsized and cotton-cloth-derived disordered carbon tubes (DCT). 108he coating over the solid electrolyte effectively reduced interfacial resistance.An improved sodium wetting of the DCT-modified solid electrolyte enables uniform and fast Na ion transport across the interface.The coating significantly reduced the interfacial resistance from 750 to 150 Ω cm −2 as compared to that of the symmetrical cell.Even after 400 cycles, the interfacial resistance rose to ∼200 Ω, whereas the cell without a DCT coating exhibited a high interfacial resistance of ∼2000 Ω.
Employment of a Wetting Agent.Ionic liquid and organic liquid electrolytes have also been used as wetting agents in addition to solid composite electrodes to improve the intimate contact between the cathode and solid electrolyte.The ionic liquid is preferred because of its nonflammability, nonvolatility, high thermal stability, and electrochemical stability since the liquid electrolyte is unstable at high temperatures.Gu et al. proposed a strategy of adding a small amount of liquid electrolyte (5 μL of 0.8 M NaPF 6 salt in ethylene carbonatedimethyl carbonate (EC-DMC)) and a nonvolatile and nonflammable ionic liquid (i.e., N-methyl-N-propylpiperidinium-bis(fluorosulfonyl) imide (PP 13 FSI, ∼5 μL cm −2 )) as a wetting agent and achieved a significantly reduced interfacial resistance as shown in Figure 10(h and i). 109For the AS 3 B configuration, Na 3 V 2 (PO 4 ) 3 /liquid electrolyte/ Na 3.3 Zr 1.7 La 0.3 Si 2 PO 12 /Na and Na 3 V 2 (PO 4 ) 3 /ionic liquid/ Na 3.3 Zr 1.7 La 0.3 Si 2 PO 12 /Na batteries with different loadings of cathode materials (1.67 and 3.33 mg cm −2 ) were tested for AS 3 B. The interfacial resistance of Na 3 V 2 (PO 4 ) 3 /LE/ Na 3.3 Zr 1.7 La 0.3 Si 2 PO 12 /Na was much lower than that of Na 3 V 2 (PO 4 ) 3 /Na 3 .3 Zr 1 .7 La 0 .3 Si 2 PO 1 2 /Na.However, Na 3 V 2 (PO 4 ) 3 /ionic liquid/Na 3.3 Zr 1.7 La 0.3 Si 2 PO 12 /Na exhibited interfacial resistance comparable to that of conventional liquid electrolytes. 30,112he inclusion of ionic liquid in the cathode materials will facilitate faster Na + transport via the space−charge layer and the electric field induced by the ionic liquid layer.The as-fabricated AS 3 B delivered initial charge and discharge capacities of 116 and 113 mA h g −1 , respectively, at a current of 0.2 C. The specific discharge capacities were 113, 112, 109, 106, 103, 97, 91, and 86 mA h g −1 at cycling rates of 0.2, 0.5, 1, 2, 4, 6, 8, and 10 C, respectively, as shown in Figure 10(j and k).The excellent electrochemical performance demonstrated by Na 3 V 2 (PO 4 ) 3 / ionic liquid/Na 3.3 Zr 1.7 La 0.3 Si 2 PO 12 /Na-based AS 3 B was attributed to the high σ of the composite solid electrolyte and the addition of a small amount of ionic liquid at the electrolyte− electrode interface.In another reported work, Jung and Hong et al. obtained AS 3 B by employing an Na 3 SbS 4 -coated NaCrO 2 Langmuir cathode with a coating thickness of 200 nm.The as-fabricated Na 3 SbS 4 -coated NCO/Na−Sn cell successfully delivered a discharge capacity of 108 mA h g −1 , like that of the liquidelectrolyte cell. 110ormation of an Interlayer between Electrode− Electrolyte Interfaces.It has been demonstrated that creating an interlayer or interphase for the ions to be transported between solid electrolytes and cathodes can lower the interfacial resistance.Similar to the liquid electrolyte−Na metal system, SSE also forms three types of interlayers.Type 1 is the thermodynamically stable interface, which does not undergo electrochemical or chemical reactions.Type 2 is the nonpassivizing layer with mixed electronic and ionic conducting pathways (MCI), and type 3 is a stable solid electrolyte interface (SEI), which provides a favorable interface with a high σ and negligible electronic conductivity.Types 1 and 3 are highly recommended for the stable performance of the solid state sodium metal battery.Type 2 results in detrimental dendritic growth, which causes improper contact between the SSE and metal anode.Furthermore, it can be solved by melting sodium metal onto the surface of inorganic SSE.Doping and substitution in the inorganic SSE help to improve the cycling efficiency and also promote the affinity toward sodium metal.Another viable technique is to mitigate the interlayer compatibility by constructing electronically insulating artificial SEI on the surface of the Na-metal anode. 113nterfacial issues in AS 3 B during charge−discharge cycles may be extensively overcome by introducing a stable interstitial interlayer that is wetted by the anode and should be a good Naion conductor.In this direction, Goodenough et al. proposed an AS 3 B with a dendrite-free anode with a reduced anode/ceramic interfacial resistance by the introduction of an in situ-formed thin Na + conductive interfacial layer and an interstitial dry polymer film at the interface.The formation of an interlayer results in the suppression of dendrite growth because the grain boundaries associated with ceramic pellets are not in direct contact with the sodium anode, as depicted in Figure 11(a and  b). 114The Na/Na symmetrical cell fabricated with a heated Na 3 Zr 2 Si 2 PO 12 (H-Na 3 Zr 2 Si 2 PO 12 ) IE (i.e., Na/H-Na 3 Zr 2 Si 2 PO 12 /Na) exhibits an interfacial resistance from 4000 to 400 Ω cm −2 as compared to Na/Na 3 Zr 2 Si 2 PO 12 /Na.Further effective dendrite suppression was assessed by using time-dependent voltage profiles under a current density of 0.15 mA cm −2 and then increasing to 0.25 mA cm −2 .In this context, Na/H-Na 3 Zr 2 Si 2 PO 12 /Na demonstrated stable sodium plating−stripping cycles for up to 550 h, but Na/Na 3 Zr 2 Si 2 PO 12 /Na displayed a short circuit within 1 h when subjected to a current density of 0.15 mA cm −2 due to the rapid formation of dendrites, causing a penetration of the grain boundaries as depicted in Figure 11(c−f).This is attributed to the excellent wetting of the interlayer that delivers a more uniform sodium flux across the cathode−electrode interface.On the other hand, an interlayer of cross-linked poly(ethylene glycol) methyl ether acrylate (CPMEA) was introduced on both sides of the NASICON electrolyte in the symmetrical cell with a configuration of Na/ CPMEA/H-Na 3 Zr 2 Si 2 PO 12 /CPMEA/Na. 102The as-prepared symmetric cell exhibits a stable voltage profile for up to 380 h when subjected to a current density of 0.20 mA cm −2 , revealing the growth suppression of dendrites.

■ ANODE−ELECTROLYTE INTERFACE
Owing to the low melting point and mechanical modulus, a sodium metal anode is highly recommended to develop wettable surfaces.Despite such advantages, further progress of AS 3 B is plagued by the issue of high interfacial charge transfer resistance and poor interfacial stability. 115,116Moreover, the high chemical reactivity of sodium metal across the anode−electrolyte interface raises a serious concern for developing AS 3 B.In particular, in AS 3 B, an ideal surface is characterized by the interface between the Na anode and solid electrolytes, facilitating rapid ion transport with sacrificial dendrite formation during the plating/stripping processes.Over several charge/ discharge cycles, insignificant volume change in the sodium metal anode does not provide any intimate contact across the rigid electrolyte interface, causing a decay in the cyclic stability performance of the AS 3 B. Thus, strategies focusing on the interfacial modifications (structure stabilization, electrolyte optimization, improving contact by mechanical methods, and  interface engineering via chemical methods) across the anode− electrolyte interface are highly urged to overcome the existing IE's to improve the electrochemical and device performance of AS 3 B. 117 The strategies to improve the anode−electrolyte interfacial contact are as follows: Structure Stabilization.Sodium alloy-based anodes are attracting immense attention owing to their high gravimetric (volumetric) specific capacities and adequate Na inserting potential, qualifying them as potential anode candidates for high-energy AS 3 B. The Na alloy as compared to Na metal anodes serves as a better candidate for AS 3 B. 115,117 The host for the Na metal anode is primarily required to bear the volume change during the plating/stripping processes, as the Na anode is rigid and unable to deform after considerable volume change.A Na alloy, on the other hand, could act as an electron and ionconductive host because its framework can provide continuous pathways for transporting electrons and Na ions, allowing for Na stripping and plating as well as maintaining seamless interface contact with SSEs, resulting in a morphology bereft of dendrites.
Furthermore, due to their lower reactivity with SSEs compared to that of the Na metal, the alloys can form a stable interface.Despite their superior advantages, alloys are prone to irreversible capacity, huge capacity fading, and poor cycling stability.Recently, large emphasis has been placed on developing alloy-based storage materials.These anodes mainly comprise Na with the elements belonging to groups IVA and VA (i.e., Sn, Sb, Ge, Bi, Si, and P) in the periodic table due to their high theoretical specific capacities.The sodiation parameters of widely reported alloys are given in Table 2.
The Ge-based electrodes are widely preferred in rechargeable batteries owing to their impartial volume swelling.The theoretical specific capacity of the Na−Ge binary phase exhibits a large volumetric capacity of 1974 mA h g −1 , which is much higher as compared to Ge having a volumetric capacity of 369 mA h g −1 .The volume swelling of Ge is superior to that of Sn, Sb, and P. 118 Yu et al. prepared multi-core−shell-structured Bi@Ndoped carbon-based electrodes to encounter the structural degradation and instability of SEI at the electrode−electrolyte interface during charge/discharge. 119The encapsulated Bi spheres encapsulated by a conductive porous carbon shell contributed to the enhanced electrochemical activity and electrical conductivity of AS 3 B. First, the complete encapsulation of Bi nanospheres in carbon precludes volume changes during the charging/discharging, resulting in SEI formation on the outer surface rather than Bi nanoparticles.On the other hand, the nanosized Bi decreases the diffusion length for both ions and electrons, resulting in a better rate capability and power performance.The as-prepared Bi@N-C composite exhibits an outstanding rate capability of 178 mA h g −1 at 100 A g −1 with a ultralong cycle life of 235 mA h g −1 at 10 A g −1 after 2000 charge/discharge cycles for AS 3 120 The introduction of Na 15 Sn 4 into the Na matrix enhanced the Na + diffusivity on the anode side that suppresses the pore formation at the anode− electrolyte interface.Consequently, the symmetrical composite anode cell demonstrated excellent performance (i.e., a current density of 2.5 mA cm −2 and stable galvanostatic cycling for more than 500 cycles at 0.5 mA cm −2 ). 118Yue et al. assembled a AS 3 B with a Na−Sn alloy with acetylene black as the anode, Na 3 PS 4 as SSE, and SSE-coated Mo 6 S 8 as the cathode, which also resulted in an improved interfacial contact between the SSE and anode. 93ao et al. established a strong bulk-hybrid Na metal anode employing a melt infusion procedure that combined the molten Na with the surface-modified Na 3.4 Zr 2 Si 2.4 P 0.6 O 12 (NZSP).A conformal SnO 2 layer was precoated on the surface of the NZSP particles to enhance the affinity with the molten Na.SnO 2 can react with Na to create a Na−Sn alloy during heating, considerably improving the Na wettability.Across the composite anode, a rapid and continuous channel for the simultaneous transport of electrons and Na is formed.Because of the compact anode configuration, it delivered stable cycling for 700 h at a current density of 1 mA cm −2 with a capacity of 5 mA h cm −2 . 94ontact Improvement.Implementing a ceramic electrolyte in AS 3 Bs has the potential to overcome interfacial problems by mechanically restricting the plating of the Na metal from an external pressure, improving uniform and dense deposition and therefore ensuring the stable Na|SSE interfacial contact and high Coulombic efficiency.The mechanical environment is completely different in the context of AS 3 B, where the Na metal is compressed between a rigid metal current collector (Al, Cu, Ni plates, etc.) and SSE.The deformation of Na metal with the use of external pressure plays a crucial role in maintaining low interfacial resistance as the geometry and morphology of a Na metal anode change during charging/discharging (plating/ stripping).Recent studies have demonstrated that improving the interface contact and stifling the formation of voids may be accomplished by adding an external pressure (i.e., stack pressure).According to Kasemchainan et al., the constant electrochemical cycling seen in Figure 12(a−e) can be advantageous for Li metal stripping and plating at a stacking pressure of 5−7 MPa.The contact areas of Na and Li interfaces are 70.7 and 13.1% after 2 h of creep.By increasing the loading time, the creeping of the contact regions increases while the noncontact areas also come into contact with the solid electrolyte surface.These new areas start to carry loads and release local stresses to previously loaded areas as shown in Figure 12d.Furthermore, the rate of contact fraction is greater for Na metal than Li metal (Figure 12e). 41Zhang et al. proposed a 3D time-dependent model for observing the development of interfaces formed between Na metal and Na-β″-Al 2 O 3 . 99he differences due to the contact elastoplasticity are greater than the differences in metal creep effects.A more conformal contact at the high pressure caused by the increased stack pressure may result in less creeping.For example, Uchida et al. have effectively lowered the interfacial resistance of the Na/ Na 3 Zr 2 Si 12 PO 12 (NASICON) assembly to 14 Ω cm 2 at room temperature using simple mechanical compression.Their work also proved an advantage of the Na/NASICON interface over the Na-β″-Al 2 O 3 counterpart using the electrochemical impedance technique, which revealed a considerable difference in the activation energies for interfacial charge transfer. 100Electrolyte Optimization.Taking advantage of both inorganic ceramics and organic polymers, the composite electrolytes have been proposed to provide improved σ with high flexibility for reducing the interfacial resistance between solid electrolytes and electrodes. 37,122,123A large quantity of research articles have been devoted to composite electrolyte development for an improved interfacial resistance in AS 3 B.For example, Ran et al. developed a low interfacial resistance of around 572 Ω for 40 wt % poly(ethylene oxide) (PEO)-Na 3 Zr 2 Si 2 PO 12 (NZSP).Therefore, this reduced interfacial resistance delivered good mechanical properties and σ of 4.0 × 10 −5 S cm −1 as well as a good ability to suppress dendrite formation by means of their softer outer layer and the harder middle layer for all-solid-state sodium-ion batteries. 124t the same time, Yao et al. showed that polymer PEO with an embedded ceramic conductive β-alumina filler and an interfacial resistance of around 620 Ω possessed excellent interfacial stability, an improved Na-ion transference number, and a flexible electrode−electrolyte interface contact, delivering a high σ of 3.95 × 10 −4 S cm −1 at 60 °C and the widest electrochemical stability window at 5.55 V. 125 Goodenough et al. made a new revolution by the introduction of a polyethylene glycol diacrylate (PEGDA)/Na 3 Zr 2 Si 2 PO 12 /SCN composite electrolyte membrane with a superior ionic interface of the electrolyte− electrode assembly. 126For SSEs, the challenging development issue is improved interfacial resistance.Here, it is about 200 Ω with a good σ of 4.5 × 10 −4 S cm −1 .Wu et al. developed a newer polymer−ceramic matrix of PEO with P2-type layered oxide Na 2 Zn 2 TeO 6 with a nanostructured cathode of Na 2 V 3 (PO 4 ) 3 exhibiting a low interfacial area resistance of 47 Ω cm 2 and the highest σ of 1 × 10 −3 S cm −1 at 80 °C, which enabled fast sodium-ion transport across the Na/CSE interface. 127ecently, Tang et al. were the first to use a polymer−ceramic matrix with a sulfide Na 3 SbS 4 -based inorganic electrolyte, in which they obtained the least interfacial resistance of electrode/ electrolyte interface stability and reduced dendrite suppression as well as a σ of 10 −4 S cm −1 .Furthermore, sulfide-based polymer composite electrolytes are newer to all-solid-state sodium-ion batteries, and more research on this composite is needed to establish the field for large-scale development.Also, the polymer−ceramic matrix exhibited improved interfacial resistance in all forms of inorganic electrolyte optimization (βalumina, NASICON, P2-type layered oxide, sulfide-based, etc.) with both high flexibility and improved ionic conductivity in AS 3 B.
Interface Engineering.To improve the stability of the interface between SSE and Na, different functional interlayers have been investigated.Practically, intimate contact is highly desired in the case of the electrode−electrolyte interface.In addition to the crucial electrolyte threats such as reduced compactness and high electronic conductivity due to grain boundary inorganic electrolytes, dendrite growth is also induced by nonintimate contacts at the interfaces.This can be extensively done by characteristic modification techniques such as atomic layer deposition, the solvent casting method, the wet chemical method, chemical vapor deposition, and applying mechanical pressure in all-solid-state inorganic electrolytes.Furthermore, the overall view to improve the strategies for the electrode− electrolyte interface (cathode−electrolyte/anode−electrolyte) and the techniques and advantages are listed in Table 3.

■ CONCLUSION AND FUTURE OUTLOOK
All-solid-state inorganic electrolytes for AS 3 B are a trending aspect in improving battery scenarios, and this class of electrolytes inheriting superior characteristics is capable of outperforming lithium-based battery technology.The inorganic electrolytes of β-alumina (Na-β-Al 2 O 3 ) and NASICON (NZPO) exhibit moderate σ and stability at RT.Meanwhile, many dopants were utilized to significantly increase the σ of βalumina IEs, but such dopants encountered challenges in terms of their sintering temperature, resulting in a lower fractural and densification strength of the electrolyte.Similarly, in NASI-CON's solid structure, the substitution of parent atoms produces larger radii and enhances the σ.However, compared to sulfide-IEs, both IEs have low σ values and unstable interfaces in contact with the Na-metal electrode.
Furthermore, the highly conductive inorganic electrolytes employed for AS 3 B are typically chalcogenides, antiperovskites, and borates.Chalcogenides of sulfide-type inorganic solid electrolytes introduce a new glass and glass−ceramic electrolyte.Despite their air stability and handling difficulties, the cubic and tetragonal phases of sulfide-type IEs (Na 3 PS 4 ) deliver electrolytes with excellent ionic conductivity, high mechanical strength, excellent electrochemical stability, and better electrode− electrolyte interfaces than other forms of IEs.Likewise, borohydrides (Na 2 B n H n ) with large cage-like quasi-spherical architectures make several hydrides with alkali ions to form a cost-effective low-temperature method for producing high ionic conductivity.Antiperovskite IEs (e.g., Na 3 OBH 4 ) also tend to show high σ and have the advantage of low energy barriers for the characteristic Na-ion transport pathways.P2-type layered sodium-ion conductors (NZTO) exhibit a performance similar to that of NASICON, and the formation of the P6 3 22 space phase enables them to exhibit good σ and a better interface between electrode−electrolytes than other oxide-based ISEs.
Overall, chalcogenide-, antiperovskite-, and borohydridebased ISEs are becoming very competitive candidates for large-scale energy storage systems.Much more attention should be paid to the research path of these three IE factors, including salts in electrolytes and their impact upon performance, composition, and structural morphology, dissolution as well as the formation of side products, the effects of temperature and pressure, etc. for the high performance of AS 3 B.Meanwhile, for the large-scale development of AS 3 B, advanced manufacturing techniques such as roll-to-roll processing are expected to be highly studied and adopted due to their ability to efficiently produce dendrite-free interfaces.Furthermore, surface coating techniques such as atomic layer deposition and nanostructured interfaces offer precise control and improved performance at the interface, making them important areas of focus.Additionally, composite electrodes and functional interlayers provide flexibility and enhanced stability, which are also crucial for the long-term reliability of large-scale battery systems.Also, components such as separators, current collectors, binders, etc. have received less attention despite their equal contributions to quality performance.Both electrodes' structural evolution should be investigated to get a broader idea of specific capacity and capacity retention after cycles.Therefore, developing a Nabased anode along with appropriate cathode materials compatible with electrolytes used in current-generation batteries is crucial to achieving the ultimate position in the growing market.

Figure 1 .
Figure 1.Interfacial challenges and strategies for inorganic electrolytes in AS 3 B.

Figure 2 .
Figure 2. Crystal structures of (a) Na-β/β′′-Al 2 O 3 .Panel a is reproduced from ref 45.Copyright 2016 Elsevier Ltd.(b) Rhombohedral and monoclinic polymorph of Na 3 Zr 2 Si 2 PO 12 (NASICON).Panel b is reproduced with permission from ref 22.Copyright 2018 Wiley VCH Gmbh.(c) P-2-type layered electrolyte.Panel c is reproduced from ref 40.Copyright 2018 American Chemical Society.(d) Cubic and tetragonal phases of Na 3 PnS 4 .Panel d is reproduced with permission from ref 46.Copyright 2020 American Chemical Society.(e) Structure of Na 10 SnP 2 S 12 .Panel e is reproduced from ref 47.Available under a CC-BY 4.0 license.Copyright 2016 Richards et al.
β-Alumina Electrolytes.The Na-β electrolytes were initially reported by Yao and Kummer's group in 1967, realizing the practical sodium−sulfur (Na−S) batteries.The typical configuration of a Na−S battery consists of a Na anode, a β′′-Al 2 O 3 electrolyte, and a S cathode.Such early reported βalumina electrolytes are highly investigated for elevated temperature while operating rechargeable sodium−sulfur batteries.β-Alumina electrolytes exist in two types of distinguishable crystal structures based on the block layering sequences and respective chemical compositions [i.e., Na-β-Al 2 O 3 with the chemical formula of Na 2 O•8−11Al 2 O 3 ; space group P6 3 /mmc (hexagonal, a 0 = 0.559 nm, c 0 = 2.261 nm) and Na-β′′-Al 2 O 3 with the chemical formula of Na 2 O•5−7Al 2 O 3 and space group: R3̅ m (rhombohedral, a 0 = 0.560 nm, c 0 = 3.39 nm)]. 31,48Both types of crystal structures accommodate excess sodium as compared to the ideal structure of Na 1+x Al 11 O 17+x/2 (0.15 < x < 0.3), where the concentration of sodium ions is fulfilled by oxygen ions and exhibits an anisotropic Na-ion conductivity.The typical crystal structures of Na-β-Al 2 O 3 and Na-β′′-Al 2 O 3 are responsible for the ion-transport mechanism.

Figure 3 .
Figure 3. Hierarchical development of inorganic electrolytes for AS 3 B.

Figure 4 .
Figure 4. (a) Rhombohedral crystal structure of NASICON (Na 4 M 2 (AO 4 ) 3 ).(b) Local structure exhibiting characteristic cation sites in the NASICON framework.(c) Literature report regarding NASICONs having a specific element.(f) Experimentally determined ionic conductivity plot log scale of the y-axis vs x-axis Na content of NASICONs at ambient temperature and 300 °C reported based on the literature.(g) Distribution curve between theoretically (DFT) predicted Na-rich ground states/likely synthesized-NASICONs and experimentally synthesized Na-rich NASICONs (bottom panel).(h) XRD patterns of experimentally synthesized NASICONs with impurities (Hf/Zr)O 2 ) and SnO 2 , respectively.Panels a−c and f−h are reproduced from ref 54.Available under a CC-BY 4.0 license.Copyright 2021 Ouyang, B., et al.(d) Energy barriers for single-ion migration of Na + (blue color) following the Na II -Na IV -Na I -Na IV -Na III trajectory.(e) Energy barriers for Li + (purple color) having a Li II -Li IV -Li I -Li IV -Li III trajectory path.Panels d and e are reproduced from ref 56.Available under a CC-BY 4.0 license.Copyright 2022 Zhu, L., et al.

Figure 5 .
Figure 5. (a) Various chalcogenide-based electrolytes reported for AS 3 B. Panel a is reproduced from ref 58.Copyright 2020 Editorial Board of Acta Physico-Chimica Sinica.(b) Reported ionic conductivities of existing IEs for AS 3 B. Panel b is reproduced from ref 59.Copyright 2018 Elsevier Ltd.Schematic illustrations of the (c) Na Wyckoff site cage size and (d) DFT-MD Na + ion activation energy (E a ) with error bars vs d0 plot.(e) Pathway bottleneck extracted from DFT-optimized structures.(f) DFT-MD E a with error bars vs d 1 plot.Panels c−f are reproduced from ref 60.Copyright 2020 American Chemical Society.(g) Rietveld refinement of X-ray powder diffraction data and (h) crystal structure of cubic Na 2.88 Sb 0.88 W 0.12 S 4 .Panels g and h are reproduced from ref 61.Available under a CC-BY 4.0 license.Copyright 2019 Hayashi et al.
Similarly, Tateyama et al. investigated the ionic conduction mechanism of W-and Modoped Na 3 SbS 4 and the role of aliovalent cation substitution in conductivity improvement using density functional theory molecular dynamics (DFT-MD) calculations.The comparative results of W-and Mo-doped Na 3 SbS 4 revealed that the improved conductivity is accredited to the decrease in Na + activation energy.This parameter is accredited to the broadening of Na Wyckoff site cages induced by the smaller WS 4 /MoS 4 tetrahedral volume relative to the SbS 4 volume as shown in Figure 5(c−f).

Figure 7 .
Figure 7. (a) Crystal structure of Na 2−x Zn 2−x Ga x TeO 6 .(b) Charge/discharge characteristics of the cell with configuration NVP/NZTO-Gx/Na (x = 0, 0.1) with 0.2 C at 80 °C.Panels a and b are reproduced from ref 41.Copyright 2018 Wiley VCH Gmbh.(c) X-ray diffraction pattern of NZTO-Cx (x = 0−0.05)samples.(d) Enlarged view of the X-ray pattern.Panels c and d are reproduced from ref 42.Copyright 2018 Elsevier Ltd.(e) Crystal structures of P2-type and O′3-type Na 2 Zn 2 TeO 6 with corresponding ionic conductivity values.Panel e is reproduced from ref 39.Copyright 2020 American Chemical Society.

) 2 /
Pt cells exhibited an open-circuit voltage of ∼2 to 4 V vs Na/Na + (Figure6e).It displays a wide electrochemical stability window of ∼3.5 V, indicating the strong oxidative current as shown in Figure6f.The as-fabricated Na/ Na 5 (B 11 H 14 )(B 12 H 12 ) 2 /Na delivered an appreciable cycling stability for 400 h at 50 μA cm −2 , switching the current direction every 1 h as shown in Figure6g.Recently, Guo et al. developed a high-voltage hydroborate-based Na 3 B 24 H 23 -5Na 2 B 12 H 12 solid electrolyte battery of 4 V that features a high σ of 1.42 mS cm −1 , a high transference number of 0.97, and a 6 V wider electrochemical stability window, which is known to be higher than that of the other IEs for AS 3 B.65 Antiperovskite-Based Electrolytes.Antiperovskite-based electrolytes such as NaMgF 3 , (K,Na)MgF 3 , Na 2.9 Sr 0.05 OBr 0.6 I 0.4 , Na 9 Al(MoO 4 ) 6 , Na 3 OBr, and Na 4 OI 2 are newly emerged inorganic solid electrolytes for AS3 B with an improved σ.66 A widely investigated Na 3 OBr exhibits a cubic antiperovskite-type chemical structure (space group Pm3m, a = 4.5674(1) Å).In this structure, corner-sharing ONa 6 octahedra with Br − ions are located on the A site of the crystal framework.On the other side, Na 4 OI 2 exhibits a tetragonal antiperovskite crystal structure (space group I4/mmm, a = 4.6729(1) Å, and c = 15.9556(5)Å).Based on the theoretical calculation, both Na 3 OBr and Na 4 OI 2 show low activation energies for the characteristic Na + -ion migration pathways. 67P-2-Type Layered Oxide-Type Electrolytes.In 2011, Evstigneeva et al. introduced a class of P-2-type layered Nabased compounds with the stoichiometric chemical formula of Na 2 M 2 TeO 6 (M = Ni, Co, Zn, Mg).They first reported the crystal structure of Na 2 Ni 2 TeO 6 and Na 2 Zn 2 TeO 6 , revealing that Na 2 M 2 TeO 6 (M = Zn, Co, Mg) is constituted of MO 6/3 and TeO 6/3 octahedral units ordered in the plane along with shared edges. 36Interestingly, Na 2 Ni 2 TeO 6 exhibited the space group P6 3 /mcm (a 0 = 5.2074 Å, b 0 = 11.1558Å), and Na 2 Zn 2 TeO 6 crystallizes into space group P6 3 22 (a 0 = 5.2796 Å, b 0 = 11.2941Å)

Figure 8 .
Figure 8. Ionic conductivity and electrochemical stability window of different IEs for AS 3 B.Figure 8 is reproduced from ref 29.Copyright 2023 Wiley VCH GmbH.

Figure 8
Figure 8. Ionic conductivity and electrochemical stability window of different IEs for AS 3 B.Figure 8 is reproduced from ref 29.Copyright 2023 Wiley VCH GmbH.

Figure 9 .
Figure 9.Recently reported electrode materials: (a) cathodes and (b) anodes for AS 3 B.Figure 9 is reproduced from ref 99.Copyright 2017 Royal Society of Chemistry.

Figure 9
Figure 9.Recently reported electrode materials: (a) cathodes and (b) anodes for AS 3 B.Figure 9 is reproduced from ref 99.Copyright 2017 Royal Society of Chemistry.

Figure 10 .
Figure 10.(a) Schematic diagram of AS 3 B with device configuration Na 4 C 6 O 6 /Na 3 PS 4 /Na 15 Sn 4 .(b) SEM image of catholyte cross-sectional interface and cathode surface with EDX mapping.(c) Charge−discharge characteristics of fabricated AS 3 B at various cycle numbers at 0.1 C at 60 °C.(d) Charge−discharge characteristics at different current rates.(e) Plot of capacity and Coulombic efficiency with respect to cycle number at 0.2 C at 60 °C.Panels a−e are reproduced from ref 111.Copyright 2018 Wiley-VCH GmbH.(f) Unfavorable interface between oxide cathode and Na 3 PS 4 .(g) Favorable electrode−electrolyte interface between the organic cathode and Na 3 PS 4 electrolyte.Panels f and g are reproduced from ref 112.Copyright 2019 Elsevier Ltd. (h and i) Schematic diagram of interfaces (Na 3 V 2 (PO 4 ) 3 /Na 3.3 Zr 1.7 La 0.3 Si 2 PO 12 /Na and Na 3 V 2 (PO 4 ) 3 /ionic liquid/ Na 3.3 Zr 1.7 La 0.3 Si 2 PO 12 /Na).(j and k) Cycling performance and Coulombic efficiency characteristics of cell Na 3 V 2 (PO 4 ) 3 /Na 3.3 Zr 1.7 La 0.3 Si 2 PO 12 / Na and Na 3 V 2 (PO 4 ) 3 /ionic liquid/Na 3.3 Zr 1.7 La 0.3 Si 2 PO 12 /Na at room temperature at 10 C for 10 000 cycles.Panels h−k are reproduced from ref 109.Copyright 2016 Wiley-VCH GmbH.

Figure 11 .
Figure 11.Schematic diagram showing the contact model of IE and the Na metal-based anode: (a) a poor wetting ability ceramic pellet and (b) a good wetting ability artificial interlayer during the plating of sodium.(c and d) Nyquist plots of the device with configuration Na/NASICON/Na and Na/H-NASICON/Na symmetric cell, respectively, at 65 °C.(e) Capacity versus voltage curve of a Na/H-NASICON/gold foil at a scanning rate of 0.5 mV s −1 and (f) cycling stability test of the Na/H-NASICON/Na symmetric cells at 65 °C.Figure 11 is reproduced from ref 102.Copyright 2017 American Chemical Society.
Figure 11.Schematic diagram showing the contact model of IE and the Na metal-based anode: (a) a poor wetting ability ceramic pellet and (b) a good wetting ability artificial interlayer during the plating of sodium.(c and d) Nyquist plots of the device with configuration Na/NASICON/Na and Na/H-NASICON/Na symmetric cell, respectively, at 65 °C.(e) Capacity versus voltage curve of a Na/H-NASICON/gold foil at a scanning rate of 0.5 mV s −1 and (f) cycling stability test of the Na/H-NASICON/Na symmetric cells at 65 °C.Figure 11 is reproduced from ref 102.Copyright 2017 American Chemical Society.

Figure 12 .
Figure 12.(a) Creep evolution of the sodium−solid electrolyte and lithium−solid electrolyte contact areas with time subjected to a stack pressure of 1.0 MPa.(b) Contact stress maps of the Na surface.(c) Li surface at loading for 0, 0.1, and 2 h.(d) Schematic diagram for the interfacial creep process.(e) Rates of increase in contact fraction with time of the Na-SE and Li-SE interfaces.Figure 12 is reproduced from ref 121.Copyright 2021 American Chemical Society.
Figure 12.(a) Creep evolution of the sodium−solid electrolyte and lithium−solid electrolyte contact areas with time subjected to a stack pressure of 1.0 MPa.(b) Contact stress maps of the Na surface.(c) Li surface at loading for 0, 0.1, and 2 h.(d) Schematic diagram for the interfacial creep process.(e) Rates of increase in contact fraction with time of the Na-SE and Li-SE interfaces.Figure 12 is reproduced from ref 121.Copyright 2021 American Chemical Society.

Table 2 .
Sodiation Parameters of Na-Based Alloy Anodes